Process for coating metals and product thereof



UnitcdStates Patent Q PROCESS FOR COATING lVIETALS' AND PRODUCT THEREOF John E. Castle, Hockessin, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del'., a corporation of Delaware No Drawing. Application septen ller;Z1,1954 Serial No. 457,574

14 Claims. (Cl'. 148-614) This invention relates to the-protectionofmetalliosurfaces against corrosion. More particularly, it r'elatesto preventing the corrosion of ferrous metals by treating the surfaces thereof with fer-rates.

Among the methods available; for protecting metals against corrosion are several in which activechemicals: are applied to the surface of the metal to form. aninsulating coating thereon. Such a, coating prevents contact between unreacted metal and the atmosphere or' moisture and thus eliminates the major'causes of corrosion, In one chemical treatment, for example, a fer-- ro-us base is coated with a layer of inorganic phosphates by immersion in a solution containing metal ions and phosphoric acid. In another chemical treatment, steel is oo y oxida io n a al bot This invention has, as its object, provision-. of a1 noyel The abovementioned and further objectsmaybe;

achieved in accordance with the principles -ofthis invem tionby a process inwhich a protective coating is laidon; a metallic workpiece by contacting the surface- Qfthe; metal with anaqueous solution; containing; a ferrate. When the workpiece is made-from aferrous-metal; such treatment; produces thereon a thin, yellow, iron-c0ntain-- ing film solublein most acidsfil i; impervious, for; some; time to penetration, by the atmosphere and; industrial gases. This film may be termed,a; ferratesfilm-althoughits physical structure and chemical composition are not precisely known.

Th o at s o h s n ntion. a ohexavalen i on..- ompounds and may be prepared by the method of, Thompson-- et at, J. Am. Chem. Soc. 73, 1379 .1951), In this procedure the iron in a trivalent iron compound isoxidi zed by, hynochlorite tothe hexavalent state. Sodium ets, N zF ot, a i y p epar zby he. xi ation of. ferric nitrate and other ferrates may. be n 1 a de-from the; sodium compound The preferred ferrates-ofthe-invention, are the alkali metal salts, particularly those: ofso:- ium. andp tas i m- Theconcentr i n f. hev erre s. mp yed-i no n rticularly critical. Atrleasttl iby. weight shouldhowever, be utilized for; goodresultswhile up,to.1,% is desirable. The lower concennationsappear to give less pro; ti n h n h igh r u -prote tion may e: ted; at concentrations. as low. as .0, ,3% At, Concentrations: mu h ghe n e mpo t on: of: the: tenets. be comes greatly accelerated.

Since, the ferrates I readily decompose ywith theevolution of oxygen, the solutions;utilizedshouldipreferably: be freshly-prepared. They may, however; bewstabilized for some time by theinclusion therein of an orthoph'o's phate, non-cyclic triphosphate or tetraphosphate-ian-at a pH- of around 9. Alternatively, stabilization may be achieved by use of a solution having a pH of 11 or above.

' C., their resistance to corrosion gradually-increases;

2,850,415 Patented Sept. 2, 1958 ice A pH below 7 should be avoided since acid conditions greatly accelerate the decomposition of the ferrates.

Conventional buflers may be employed to. maintain the ferrate solutions alkaline at all but very high pI-Is. At, pH 11 the anti-corrosive effect of the solutions is surprisingly found to be greater at highv buffer concentrations than at low. Thus, a ferrate solution one tenth molar in respect to both sodium carbonate and bicarbonate, adjusted to pH 11 with NaOH, is more efiective in inhibiting corrosion than one containing less ofthe butfering compounds. Still better results: are obtained from a ferrate solution almost saturatedwiththe buffer.

Temperature seriously affects the stability of ferr-ates but has little influence on the speed of coating or the effectiveness of the coat produced. Temperatures above 40+5Qs G; should be avoided. because of the: increased rate of decomposition. Normally, thesolutions will be made upand utilized at ambient or room temperature but. ice, or. oven. lower temperatures can beneficially be utilized;

At: room. temperature, for best results the time of'contactbetween the workpiece being treated and theoxidizing solution should be about one hour.

Theprocess of this invention is not restrictedforuse with ferrous metals such as cast iron. or steel; Itmay be applied to other commercial metalssuch as aluminum, titanium, lead and zinc. Infact, ferrates reactwith-many' metals to form oxide films thereon and: these films may aiford some protection. against corrosion. Since Zincand aluminum, inparticular, are amphoteric, a pH within the range-Q-lzl isdesirable in dealing with these metals:

Tzhefcrrate-films ofthisinvention can be improved to someextent: by baking them. Thus, if they are heated iii-nitrogen forone or two hoursat around 200'--300 The optimum temperature for: the heating is about 250 C. Oxygen and hydrogen may destroy-the films and should beavoided during the baking period; Gases providing truly. inert: atmospheres such as the noblegases, argon, helium, neon and others, may, however, be substituted for: the:nitrogen. Ferrate filmstreated as described here maysbe referred to as baked films.

It is generally desirable to pretreat the metal on which a-ferrate coatingis to be laid down. Thus, a steel workpiece may first be degreased, if necessary; pickled'for a short: time in'hydrochloric acid, rinsed and neutraliied' for a: few'minutes in dilute caustic containing a littlecyani'de. The cyanide is commonlyused' to suppress" rustingon wet ferrous surfaces exposed for shortperiods to the-atmosphere.

A- preferredembodiment of the invention'utilizing'the principles noted above may be described as follows: A

ferrous workpiece is carefully degreased and pickled to' present a-bright surface and neutralized with dilntecyanide-solution; The workpiece is then immersed for'at least one hour at ambient temperatures in a-strongly'alkaline aqueous ferrate solution preferably having a pH of 11 or above and most desirably containing a high concentration of buffer, i. e., the solution should be .at .least'one normal in respect to buffering agents. The fe'rrate concentration should be about 1%. withdrawn, rinsed and dried, possessing a' thin yellow coat soluble in acids but affording good protection against.

amples, was prepared by the method of Thompson. et al.',

loc. cit;, modified slightly. 240 grams of sodium hydroXide was dissolved in 600 ml. of Water at 510 C.,

The. workpiece is then a temperature maintained until the ferrate was washed with ethanol. 160 grams of chlorine was passed in, 560 grams of sodium hydroxide added and the mixture centrifuged. 200 grams of powdered iron (III) nitrate enneahydrate was added and then 200 additional grams of sodium hydroxide. The mixture was again centrifuged and 800 ml. of saturated potassium hydroxide solution added to the liquid. The precipitated potassium ferrate was filtered 'on coarse sintered glass and leached with twenty 20 ml. portions of 3 N aqueous potassium hydroxide. rated potassium hydroxide solution. The ferrate was reprecipitated by this treatment and again filtered. It was slurried once with absolute ethanol, three times with 95% ethanol, each time for twenty minutes, and finally washed with ether and dried in dry air. This procedure gave 56 gofa brownish black solid which analysis showed to contain 73% of potassium ferrate (VI).

Example 1 This example shows the corrosion inhibiting efiect of a ferrate solution at pH 7.

.Two 3" x 1" samples of Weirtzin, a zinc-coated, hot-rolled mild steel, were pickled by immersion for ten seconds in concentrated hydrochloricacid to effect removal of'the zinc. In order to prevent corrosion before the subsequent ferrate treatment and to remove traces of acid, the samples were then immersed for four minutes in a solution containing 0.15% each of sodiumhydroxide and sodium cyanide at 75 C.

A bulfer solution was also made up, at a pH of 7, containing the following ingredients: 0.4% NaOH, 1.35% KH PO 1% Quadrafos, and more KH PO to bring the pH to 17. Quadrafos is sodium tetraphosphate, sold by the Heyden Chemical Corporation, and is used to stabilize'therferrate. The solution was diluted one hundred fold and two samples thereof chosen. To one sample was added 1% potassium ferrate prepared in the manner described above. The other sample was left blank, i. e., without ferrate.

One of the prepared steel strips was suspended at ambient temperature (about 23 C.) for four hours with one inch of its length immersed in the solution containing potassium ferrate. The other strip was contacted, under like conditions, with the blank solution. Both strips were removed from the solution, rinsed and dried, and suspended in the air-space of an air-tight 8 liter glass vessel. This vessel also contained 150 mlof water carrying dissolved 0.025 g. of sodium pyrosulfite, Na S O and two drops of concentrated sulfuric acid. The sulfur dioxide evolved passed into the air above the water and created conditions simulating those of a highly corrosive industrial atmosphere. The strips were held at 40 C. under corrosive conditions within the glass vessel for 23 hours. treated sample showed heavy uniform rusting on the end not immersed in the ferrate solution and slightly less Thus ferrate, at pH 7, oifers some corrosion protection.

Example 2 This example shows the efiect of a ferrate solution buffered at pH 9. v V

The tests of Example 1 were repeated except that the buffer solution utilized in both the blank and ferrate treatments was an aqueous solution containing 1.5% NaOH, 6.25 H 30 and 1% Quadrafos, adjusted with NaOH to pH 9 and diluted one hundred fold.

Metal strips prepared as before were immersed in the blank and ferrate solutions, rinsed and dried. They were corroded at the same time and in the same atmosphere as those of the previous example. Results were the same except that corrosion was not quite as great on the strip The filtrate was collected in 800 ml. of satu At the end of this period the ferrate j and containing 0.1%

treated with ferrate as it was on the one so treated at pH 7. p

Example 3 This example shows the effect of ferrate solutions buffered at pH 11.

(a) The tests of the previousexamples were repeated except that the buffer solution contained 10.6% Na CO 6.6% NaHCO and 1% Quadrafos, again adjusted with NaOH to pH 1] and diluted one hundred fold. Greatly improved corrosion resistance was noticeable in the strip treated at the higher pH;

(b) The test of (a) above was repeated With solution of pH 11 one-tenth molar in both Na CO and NaI-ICO Quadrafos. The increase in concentration of the buffer improved the corrosion resistance of the steel strip treated.

(c) The test of (a) was repeated except that the buffer solution was almost saturated (one molar). Results were excellent, practically no corrosion being noticeable after the 23 hours exposure of the sample treated.

' Example 4 This example shows the efliect of ferrate solutions ing a very high pH.

The tests of Example 1 were repeated except that the ferrate and blank solutions utilized for the metal treatment were ten normal with respect to NaOH and contained no buffer or Quadrafos. Results were as good in this case as in Example 3 after 23 hours exposure to the S0 containing atmosphere. r

Example 5 A 1% solution of potassium ferrate buffered at 11 with sodium carbonate, sodium bicarbonate and sodium hydroxide was used to treat 'a freshly cleaned sheet of zinc. The sheetwas suspended with'one inch of its length extending into the solution at ambient temperature for 1 hour. At the end of this period a thin yellow film was evident on the portion of its surface'contacting theferrate solution.

The treated zinc was held within a sulfur dioxidecontaining atmosphere at 40 C. for 15 hours. Less corrosion was evident on the zinc contacted with ferrate than on the untreated metal, the latter showing numerous white spots evidencing the formation of an oxide or sulfur-containing compound.

Example 6 slightly less corroded than the first; and the third sample exhibited almost no corrosion on the part treated.

Having described my invention, I claim:

1. A ferrous surface carrying a visible protective layer imparting corrosion resistance thereto formed in situ by contacting said surface with an alkaline solution containing a ferrate, thereby forming a coating on the surface,

and subsequently baking said'coating in an atmosphere substantially inert thereto.

2. The method -of imparting corrosion resistance to a metal which comprises contacting said metal with an aqueous solution of a ferrate of an alkali metal having a pH above about 7 at a temperature between about 0 and about 50 C. and thereby forming a visible protective coating on said metal and subsequently baking the havprotective coating in an atmosphere substantially inert thereto.

3. The method of claim 2 in which the pH of the solution is between about 7 and 11.

4. The method of claim 2 in which the metal is a member of the group consisting of iron, steel, zinc and aluminum.

5. The method of claim 2 in which the .concentration of the ferrate is about 1% by weight.

6. The method of claim 2 in which the time of contact between the metal and the aqueous solution is about one hour.

7. The method of claim 2 characterized in that the baking is carried out at about 200-300 C.

8. The method of claim 2 in which the pH of the aqueous solution is maintained by means of a butter.

9. The method of claim 8 in which the buffer is a mixture of the carbonates and bicarbonates of the alkali metals.

10. The method of claim 9 in which the bufier is a mixture of sodium carbonate and sodium bicarbonate.

11. The method of claim 10 in which the concentration of each of the components of the butter is between about 0.1 molar and the saturation concentration.

12. The method of imparting corrosion resistance to a ferrous metal which comprises contacting the metal for at least about one hour at ambient temperature with an aqueous solution of a ferrate of an alkali metal buffered at a pH of about 7-11 by means of a sodium carbonatebicarbonate bufier.

13. The method of imparting corrosion resistance to aluminum which comprises contacting the metal for at least about one hour at ambient temperature with an aqueous solution of a ferrate of an alkali metal buffered at a pH of about 711 by means of a sodium carbonatebicarbonate buffer.

14. The method of imparting corrosion resistance to zinc which comprises contacting the metal for at least about one hour at ambient temperature with an aqueous solution of a ferrate of an alkali metal buffered at a pH of about 7-11 by means of a sodium carbonatebicarbonate butler.

References Cited in the file of this patent UNITED STATES PATENTS 1,440,092 Mai Dec. 26, 1922 2,301,983 Tanner Nov. 17, 1942 FOREIGN PATENTS 218,619 Great Britain Apr. 29, 1924 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chem, vol. 13, pages 930933.

Transactions of American Electro-Chemical Society, 29, page 251 (1916).

Thorpes Dictionary of Applied Chemistry, vol. II (B), page 123. 

2. THE METHOD OF IMPARTING CORROSION RESISTANCE TO A METAL WHICH COMPRISES CONTACTING SAID METAL WITH AN AQUEOUS SOLUTION OF A FERRATE OF AN ALKALI METAL HAVING A PH ABOVE ABOUT 7 AT A TEMPERATURE BETWEEN ABOUT 0* AND ABOUT 50*C. AND THEREBY FORMING A VISIBLE PROTECTIVE COATING ON SAID METAL AND SUBSEQUENTLY BAKING THE PROTECTIVE COATING IN AN ATMOSPHERE SUBSTANTIALLY INERT THERETO. 